An electron microscope irradiates a material to be observed (sample) with an electron beam emitted from an electron gun while controlling the electron beam using an electron optical system such as an electron lens and a deflector. Then, the principle of the electron microscope is to detect transmission electrons transmitted through the irradiated sample and reflection electrons and secondary electrons generated by the interaction between the sample and the electron beam and thus observe the sample in an enlarged manner. The electron gun which generates the electron beam plays an important role in this electron microscope.
In a typical structure of the electron microscope, the electron gun which generates the electron beam is provided so as to be combined with an irradiation system, an electromagnetic lens such as an objective lens, and a detection device for the electron beam. Here, the electromagnetic lens has a function of transporting and converging the electron beam to irradiate the sample therewith.
In order to obtain a clear observation image with high resolution for a short time, it is necessary to irradiate the sample with a bright electron beam (large irradiation current) being converged on a small spot on the sample. At this time, in order to form a small and bright electron beam spot on the sample, it is necessary to increase the brightness of the electron gun. Here, the brightness of the electron beam is defined as the amount of current per area per solid angle of a light source, and in the principle of electron optics, the brightness of the electron beam which is transported by the electromagnetic lens without changing its energy cannot exceed its original brightness. For this reason, a high-brightness electron gun is necessary to obtain an electron microscope with higher brightness.
A cold cathode field-emission (C-FE) electron gun is widely used as such a high-brightness electron gun for an electron microscope having high resolution. According to the principle of electron beam generation of this electron gun, a strong electric field is generated in a leading end part of a tungsten single crystal which is thinly sharpened by electric field polishing, and an electron beam is extracted by the strong electric field. The cold cathode field-emission electron gun is closer to a point light source than other types of electron sources, and thus can provide a high-brightness electron beam. In addition, the cold cathode field-emission electron gun can provide an electron beam in which energy fluctuations (energy width ΔE) of each electron in the extracted electron beam are small.
Up to now, widely used is an electron gun having the structure obtained by combining this cold cathode field-emission electron gun and an electrostatic lens structure which is put into practical use by Butler and others in 1966.
FIG. 1 illustrates the structure of a typical cold cathode field-emission electron gun including a Butler-type electrostatic lens. A potential difference (V1) between an electron source 101 and an extraction electrode 110 is applied by an extraction power supply 105, and this forms an electric field in a leading end part of the electron source 101 (a lowermost part of the electron source 101 in the figure). Field emission is caused at the electron source 101 by the formed electric field, and an electron beam is emitted. Part of the emitted electron beam passes through an aperture 109 provided in an extraction electrode 110, and is converged by the electrostatic lens action of an electric field formed by Butler-type electrodes 103 and 104 provided between the extraction electrode 110 and an anode 107. At the same time, the part of the emitted electron beam is accelerated by a potential difference (V0−V1) between the extraction electrode and the anode (when the magnitude of the acceleration potential |V0|> the magnitude of the extraction potential |V1|) or is decelerated (when |V0|<|V1|), to be emitted to a sample.
This structure can easily make the structure of the electron gun relatively small, and thus is advantageous to achieve an ultrahigh vacuum. In addition, this structure is advantageous because the electrostatic lens enables both the acceleration and convergence of the electron beam at the same time.
Unfortunately, in this electron gun, the total amount of current which can be taken out is smaller than that of other types of electron sources such as a Schottky electron source, and if a large current is to be taken out, it is necessary to use an electron beam which is emitted at a wide angle from a chip. For example, this electron gun is disadvantageous in that, if several percent of the total amount of current emitted from the chip (cathode) is to be taken out, the brightness (effective brightness) considerably decreases due to an influence of the aberration of the electrostatic lens. As a larger amount of current is to be taken out, this phenomenon occurs more remarkably. Such aberration of the electrostatic lens is difficult to reduce. For this reason, in an analytical electron microscope which requires a high irradiation current for the purpose of elemental analysis or other analyses, the brightness and the energy width are relegated to second place, and another electron source such as a Schottky electron source (hot cathode field-emission electron gun) is used in many cases.
Meanwhile, for the conventional hot cathode field-emission electron gun, an electron gun which converges an electron beam using a magnetic field lens has been devised for the purpose of enhancing the brightness of the electron gun.
In terms of a reduction in aberration, a larger number of structures (referred to as an immersion type) in which an electron source is provided inside of a magnetic field have been devised than such a structure as disclosed in Patent Literature 1 in which a converging lens formed by a magnetic field is provided immediately below an electron gun, and there are known examples of the detailed structure of the immersion type as disclosed in Patent Literatures 2 to 7.
The structures disclosed in these known examples generally concern a hot cathode field-emission electron gun, and thus have a great difference from the structure of a cold cathode field-emission electron gun in whether or not a suppressor is provided. The suppressor which is characteristically provided in the hot cathode field-emission electron gun has a function of reflecting thermal electrons emitted from a heated filament adjacent to an electron source and confining the thermal electrons to the suppressor with a negative potential being applied to the electron source.
In contrast, a filament is not heated in the cold cathode field-emission electron gun, and thermal electrons are not emitted, so that the suppressor is unnecessary. In the cold cathode field-emission electron gun, an electrode provided adjacently to an electron source is only an extraction electrode, and the extraction electrode serves to generate a large electric field in a leading end part of the electron source, to thereby cause field emission from the leading end.
Because there is such a difference in structure, an optimal structure is considerably different between the hot cathode field-emission electron gun and the cold cathode field-emission electron gun, even in an immersion-type electron gun including a magnetic field lens. For reasons to be described later, even if the structures of the known examples are applied to the cold cathode field-emission electron gun without any change, high performance cannot be achieved.